Novel Pyrazole Acyl(thio)urea Derivatives Containing a Biphenyl Scaffold as Potential Succinate Dehydrogenase Inhibitors: Design, Synthesis, Fungicidal Activity, and SAR

As part of a program to discover novel succinate dehydrogenase inhibitor fungicides, a series of new pyrazole acyl­(thio)­urea compounds containing a diphenyl motif were designed and synthesized. Their structures were confirmed by 1H NMR, HRMS, and single X-ray crystal diffraction analysis. Most of these compounds possessed excellent activity against 10 fungal plant pathogens at 50 μg mL–1, especially against Rhizoctonia solani, Alternaria solani, Sclerotinia sclerotiorum, Botrytis cinerea, and Cercospora arachidicola. Interestingly, compounds 3-(difluoromethyl)-1-methyl-N-((3′,4′,5′-trifluoro-[1,1′-biphenyl]-2-yl)­carbamoyl)-1H-pyrazole-4-carboxamide (9b, EC50 = 0.97 ± 0.18 μg mL–1), 1,3-dimethyl-N-((3′,4′,5′-trifluoro-[1,1′-biphenyl]-2-yl)­carbamoyl)-1H-pyrazole-4-carboxamide (9a, EC50 = 2.63 ± 0.41 μg mL–1), and N-((4′-chloro-[1,1′-biphenyl]-2-yl)­carbamoyl)-1,3-dimethyl-1H-pyrazole-4-carboxamide (9g, EC50 = 1.31 ± 0.15 μg mL–1) exhibited activities against S. sclerotiorum that were better than the commercial fungicide bixafen (EC50 = 9.15 ± 0.05 μg mL–1) and similar to the positive control fluxapyroxad (EC50 = 0.71 ± 0.11 μg mL–1). These compounds were not significantly phytotoxic to monocotyledonous and dicotyledonous plants. Structure–activity relationships (SAR) are discussed by substituent effects/molecular docking, and density functional theory analysis indicated that these compounds are succinate dehydrogenase inhibitors.